1. Field of the Invention
This invention relates to turbomachinery rotor components and assemblies having discontinuities about which form stress concentrations during rotor operation, often referred to as stress risers and, more particularly, to turbomachinery rotor components having localized compressive residual stress zones imparted in the areas of these stress risers by laser shock peening.
2. Description of Related Art
Turbomachinery such as gas turbine engines and, in particular, aircraft gas turbine engines operate rotor at high rotational speeds that subject the rotor's components to very high tensile stress fields particularly at radially outer edges of the rotors. These rotors are designed to operate in high tensile stress fields and often contain features that are stress risers which subject the components to fatigue failure and reduced life. In some applications (such as are in use in advanced, high tip-speed, gas turbine engines), the rim area of a rotating disk is subjected to exceptionally high stresses. The stresses include tension stresses in the circumferential direction called hoop stress and causes life limiting low cycle fatigue around areas of stress concentration, such as bolt holes, blade loading and locking slots, axial dovetail blade root slots, rabbets, and other stress risers. It is expensive to refurbish and/or replace disks and other rotor components in a gas turbine engine and, therefore, any means to enhance the rotor capability and, in particular, to extend aircraft engine component life is very desirable. The present solution to the problem of extending the life of these aircraft engine components is to design adequate margins by reducing stress levels to account for bolt hole and other stress concentration features or stress risers. This is typically done by increasing thicknesses locally, thus, adding unwanted weight to the rotor in combination with expensive features such as shaped holes and scalloped flanges. In order to ensure safe and reliable operation of older engines for extended service life, expensive redesign efforts or frequent replacement of suspect parts are required. This is expensive and obviously reduces customer satisfaction.
Blades are often mounted to the disk by dovetail assemblies which include a blade dovetail root at the radially innermost portion of a blade received in an axially extending disk dovetail slot. The dovetail slot is a stress riser as noted above. Friction occurs between opposing face portions of the dovetail root and slot during engine operation producing high dynamic stresses or stress risers which causes high cycle fatigue along areas of the disk and blade surrounding these dovetail contacting faces of the slot and the root. This problem is found mainly in fans and compressors causing cracking in the dovetail region on both blade shanks and disk posts. The cracking is found in the post below the pressure face and in the blade just in or above the pressure face both of which are areas of high stress due to centrifugal and thermal growth induced forces. This high cycle fatigue problem is also highly undesirable because of the expense to refurbish and/or replace disks and blades as well as the obvious benefit of extending the life of these aircraft engine components. The current solution to the problem, if possible, is to re-contour the dovetail slot to desensitize the design to frictional effects, remove damaged material, and decrease the high local stresses.
Another rotor component subject to cracking and failure is a splined rotor shaft coupling such as those usually employed in the shafting connecting the power turbine which must be capable of transferring large torque loading. Failure of a spline in a power turbine shaft usually results in an overspeed of the low pressure turbine rotor system resulting in uncontained blading or disk separation. The conventional method to overcome the problem of spline tooth failure is to alter the spline design and construction to account for factors such as fuel system oscillations, shaft torsional induced "wind up" causing high stress in the ends of spline teeth, shaft ellipticity, pilot eccentricity, and tooth-tooth spacing errors. This is a costly and complex method of extending the useful lives of spline couplings. There also remains a significant number of engines in the field and engine designs for which these factors cannot be taken into account because of commercial or technical reasons.
Therefore, it is highly desirable to design and construct longer lasting rotor components better able to resist both low and high cycle fatigue than present rotor components. To this end, the present invention is directed and provides the component with a region of deep compressive residual stresses imparted by laser shock peening around stress risers located in portions of the component subject to a tensile stress field due to centrifugal and thermal growth induced forces generated by the rotor when the rotor is rotating.
The region of deep compressive residual stresses imparted by laser shock peening of the present invention is not to be confused with a surface layer zone of a workpiece that contains locally bounded compressive residual stresses that are induced by a hardening operation using a laser beam to locally heat and thereby harden the work piece such as that which is disclosed in U.S. Pat. No. 5,235,838, entitled "Method and apparatus for truing or straightening out of true work pieces". The present invention uses multiple radiation pulses from high power pulsed lasers to produce shock waves on the surface of a work piece similar to methods disclosed in U.S. Pat. No. 3,850,698, entitled "Altering Material Properties"; U.S. Pat. No. 4,401,477, entitled "Laser shock processing"; and U.S. Pat. No. 5,131,957, entitled "Material Properties". Laser peening as understood in the art and as used herein, means utilizing a laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface. Laser peening has been utilized to create a compressively stressed protection layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No. 4,937,421, entitled "Laser Peening System and Method". However, the methods disclosed in the prior art do not disclose how to produce rotor components having stress risers in areas of the component subject to tensile cyclic stress fields and/or vibratory stress fields. It is to this end that the present invention is directed.